Lower oven temperature to 250°F. In mixer bowl, beat cream cheese and sugar until light and fluffy. Beat in eggs one at a time, scraping often. Beat in sour cream, vanilla and cornstarch until mixture is smooth. Place 3 cups of the batter in a small bowl; stir in melted chocolate until well blended. Using large scoop, alternately scoop plain and chocolate batters over crust. Use a wooden pick to swirl together, forming a marble effect..